Unity makes it possible to draw simple debugging information in your scene and even in-game while using the Unity Editor using Gizmos. Gizmos can be used to visualize many different things and allow you to render Cubes, Spheres, Lines and more. This is great if you want to mark points of interest, plot out your objects pathfinding or show the trajectory of projectiles in your game. The options are entirely up to you!

To add Gizmos to a MonoBehavior you can add an OnDrawGizmos function to the MonoBehavior. An example might look like this to draw a unique marker at the position of every GameObject with the MonoBehavior attached to it.

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void OnDrawGizmos() {
    Gizmos.color = Color.blue;
    Gizmos.DrawSphere(transform.position, 1);
    
    Gizmos.color = Color.white;
    Gizmos.DrawWireSphere(transform.position, 1.1f);
}

Gizmos will draw through most objects in the scene so they can be seen even when normal objects would be occluded by other objects.

By default OnDrawGizmos will always draw. However if you would like to draw Gizmos only when the object is selected in the Editor you may use OnDrawGizmosSelected in place of OnDrawGizmos. This functions the same way as OnDrawGizmos but is only called when you have selected the object.

One use for OnDrawGizmosSelected can be to change the color of the drawn object for selected objects. This code draws selected objects in white, and others in red:

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void OnDrawGizmos() {
    Gizmos.color = Color.red;
    Gizmos.DrawSphere(transform.position, 1);
}

// Invoked if Gizmos are enabled and the object is selected
void OnDrawGizmosSelected() {
    Gizmos.color = Color.white;
    Gizmos.DrawSphere(transform.position, 1);
}

More information about the functions available for drawing Gizmos in Unity: https://docs.unity3d.com/ScriptReference/Gizmos.html

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